1. Field of the Invention
This invention concerns a rotatable coupling for suspending heavy items from a cable without introducing torsional stresses into the cable when the heavy item rotates. More particularly, this invention concerns a rotatable log coupling for placement between a cable yarding haulback line and choker.
2. General Discussion of the Background
It is common practice in the logging industry to use a cable yarding system to transport cut logs over rough terrain. Such a yarding system often comprises a skyline cable suspended between a yarder tower and a stationary structure, such as a tree or tree stump. A remote control carriage rides along the skyline cable on sheaves, and a haulback cable is suspended between the carriage and yarder. The haulback cable is placed over a pulley in the carriage, and a free end of the cable is suspended from the carriage. The free end of the cable is typically provided with a hook and a plurality of chokers for attachment to logs that are to be transported. After the chokers are placed around felled trees and tightened, tension is exerted on the haulback line by the yarder to raise the felled trees above the ground. The carriage is then moved along the skyline to transport the logs over the terrain to a desired destination.
The log bundle carried by the choker imposes an eccentric load on the haulback cable from which it is suspended, resulting in rotation of the log bundle and haulback cable. This rotation in turn imposes torsional stresses within the cable that fray it. The structural integrity and strength of the cable is thereby diminished, requiring frequent replacement of the cable.
U.S. Pat. No. 2,759,234 to Brawand illustrates an early attempt to provide a rotatable log coupling which could drag logs along the ground with a cable. The Brawand coupling comprises an upper eye and a lower eye, the upper eye being attached through a swivel attachment to a main cable and haulback line. The choker is removably connected to the coupling by a ferrule which is inserted in a slot within the lower eye. Although this coupling is suitable for hauling logs across uneven ground, it is less suitable when logs are suspended in the air before being transported. Once the logs are suspended in the air, the top and bottom eyes of the coupling are free to move relative to one another, therefore allowing the suspended logs to swing like a pendulum around a horizontal axis.
U.S. Pat. Nos. 1,464,161 and 1,786,081 and 4,185,863 also disclose various rigging fittings in which a pair of ferrules are hooked into a fitting to provide a swivel connection. Use of so many ferrules is clumsy and can also be dangerous since the ferrules can become disengaged from the couplings to break the connection.
Finally, U.S. Pat. No. 2,955,855 shows a shackle for carrying a control rod in a nuclear reactor. The shackle is supported from a flexible lifting cable having a ferrule retained in a chamber in the shackle. This shackle is a unitary fixture which is not free to rotate about a vertical axis. Although it is possible that the shackle might rotate relative to the cable and ferrule from which it is suspended, frictional engagement between the ferrule and shackle would greatly limit free rotation.
It is accordingly an important object of this invention to provide a rotatable log coupling for placement between an elevated haulback line and shackle to prevent introduction of torsional stresses into the haulback line when the elevated log bundle rotates about a vertical axis.
Another important object of the invention is to provide such a rotatable coupling comprised of two portions which are free to rotate relative to each other about a vertical axis, but are not otherwise free to move relative to one another about a horizontal axis.
Yet another important object is to provide a coupling that uses only one ferrule attachment, and in which the ferrule is tightly retained within the coupling without danger of it being accidentally dislodged.
Finally, it is an important object of the invention to provide such a rotatable coupling that is suitable for use with a cable yarding operation wherein logs are elevated off the ground before being hauled to a yard.
The foregoing and other objects, features, and advantages of the invention will become more apparent from the following detailed description of preferred embodiments which proceeds with reference to the accompanying drawings.